1. Field of the Invention
The present invention relates to a non-destructive detection process which utilizes a time resolved fluorescence technique for detecting and measuring stresses and strains in solid portions of resin-molded products or coatings in a non-destructive manner, and it can be utilized effectively so as to improve the quality of resin-molded products or coatings.
2. Description of the Related Art
A large number of resin-molded products have been prepared with thermoplastic resins like polypropylene, polystyrene and nylon, and with thermosetting resins like epoxy resin and melamine resin. The resin-molded products have been used widely in kitchens as well as in vehicles such as automobiles and trains. In general, these resin-molded products are manufactured by injection molding, transfer molding or compression molding. It has been known that stresses and strains, associated with the heat developed during the forming processes, reside in the resin-molded products, and adversely affect them considerably in terms of reliability. For instance, the stresses and strains degrade the yield of the resin-molded products, and they cause defects like cracks or wrinkles in the resin-molded products after taking the finished products out of the molds. Moreover, it has been pointed out that, when the resin-molded products with stresses and strains resided therein are used for a long period of time, such resin-molded products are gradually degraded by external loads, chemicals or ultraviolet rays in terms of durability.
In addition, there occurs the case where, not only the durability of the resin-molded products, but also the durability of the coatings depends greatly on the stresses present in the coatings.
Accordingly, it is extremely important to measure stresses which act on or reside in solid portions of the resin-molded products and coatings.
There are spectroscopic methods for non-destructively measuring stresses and strains which act on solid portions or reside therein. Among the methods, a photoelasticity method, a Laser Raman spectroscopic method and an X-ray stress measurement method have been well known.
For instance, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 62-124,416, stresses and strains are measured by the photoelasticity method. The photoelasticity method is based on birefrigence which is observed when external forces are applied to material objects and stresses are produced therein. In this photoelasticity method, it is required that material objects be transparent, and that transmitted light be measured. Consequently, it is needed that solid portions applicable to the photoelasticity method be transparent.
As recited in Japanese Unexamined Patent Publication (KOKAI) No. 3-220,433, stresses and strains are measured by Laser Raman spectroscopic method. The method is based on Raman shift which appears when material objects are subjected to stresses and strains. This Laser Raman spectroscopic method has been applied so far only to specific material objects like silicon crystal and carbon fiber. Accordingly, it has been regarded impossible to apply this method to resin-molded products and coatings.
As summarized in "Non-Destructive Inspection Technique Series/Strain Measurement B" edited by the Japanese Society for Non-Destructive Inspection in 1980, the X-ray stress measurement is done by means of shift in X-ray diffraction peak (i.e., variation in interplanar spacing) which results from elastic deformation of crystal. In this X-ray stress measurement, the presence of crystal is indispensable. Consequently, it cannot be simply applied to amorphous material objects. For example, when measuring residual stresses at a predetermined portion in amorphous resin-molded products, fine crystalline particles, such as a metallic powder, should be mixed in resins in a large amount. As a result, the resins are impaired in terms of mechanical properties by the fine crystalline particles which are mixed therein in order to measure the residual stresses. Moreover, because of the fine crystalline particles, there arises a fear for varying the mechanical property of the resin itself, the magnitude of the residual stresses and the distribution thereof.